This invention relates to fluorescent lamps and, more particularly, to the shatterproofing of fluorescent lamps.
In my previous U.S. Pat. No. 3,673,401 I disclosed an arrangement in which a fluorescent lamp could be rendered shatterproof by using a cylindrical, transparent and non-frangible shield of polymeric material together with two rubber-like plastic end-caps. The cylindrical shield was made from a length of extruded plastic tubing having a diameter suitable for each size of fluorescent lamp and the end-caps were provided with a peripheral rib or flange to abut the end of the cylindrical tubing. The arrangement required hand assembly involving several steps. First, one of the end-caps was friction fitted onto the metallic ferrule at one end of the fluorescent lamp. Next, the cylindrical shield was slid over the fluorescent lamp until its end abutted the peripheral rib. Finally, the second end cap was friction fitted over the opposite metallic ferrule and its position adjusted until its peripheral rib abutted the opposite end of the cylindrical shield. Reliability of the shatterproofing depended on how carefully the four elements were put together by the user. If the fluorescent lamp were dropped or fell from its fixture so that its glass envelope broke, the shards of glass as well as the phosphorescent powders and mercury used in the lamp could all be contained. This type of shatterproof fluorescent lamp assembly became very popular in industrial settings, especially those which had to be safeguarded against contamination by toxic particulates and materials.
More recently patents have been issued directed to making the assembly hold together more securely. Thus, U.S. Pat. Nos. 5,173,637 and 4,924,368 teach that an adhesive should be applied to the exterior of the metallic ferrule of the lamp so as to cause the end cap to better adhere to the lamp. While the use of adhesive allowed greater tolerances to be employed in the fabrication of the end-cap and thus facilitated assembly as compared to using an end-cap whose inner diameter was friction-fitted to tightly embrace the metallic ferrule, the assembly operation remained a somewhat tedious hand operation requiring the lighting maintenance personnel to manually put together the elements of the fluorescent lamp protection assembly in the field rather than merely replacing burned-out lamps. It would be advantageous to eliminate the need for field assembly as well as to provide a more reliable encapsulation method.
In accordance with the principles of the present invention, as exemplified by the illustrative embodiment, a shatterproof fluorescent lamp assembly is achieved capable of containing within a polymeric envelope all of the glass, powders and mercury used in the lamp. A protective polymeric coating, advantageously a polycarbonate, is extruded directly on to the fluorescent lamp so as to be in intimately conforming embracing contact with substantially all of the contours of the lamp""s glass envelope and the ferrules at the end of the glass envelope thereby increasing the hoop strength of the glass. If the lamp is struck with sufficient force to break glass envelope, the polymeric coating will generally confine the breakage to the local area struck and, in experimental tests, the lamp will remain illuminated for a measurable period.
According to the method of the invention, the increased hoop strength of the glass envelope is achieved by passing the lamp through an air lock into the main lumen bore of an extruder crosshead which is connected to vacuum pump. A cylinder of hot, polymeric material is extruded and radially drawn inward toward the periphery of the lamp by the vacuum. The extruded cylinder should have a wall thickness, so that when cooled, it will exhibit sufficient beam strength to maintain the cylindrical shape even if the glass envelope of the fluorescent tube is shattered.
According to the preferred embodiment, prior to inserting the fluorescent lamp into the extruder crosshead, the fluorescent lamp is wiped down to remove any dust. Advantageously a plastic end cap may be slipped over the ferrule at the end of the fluorescent lamp to cover the vent holes which certain types of fluorescent lamps exhibit. Alternatively, a short length of easily removable silicone tubing may be fitted over the electrical terminals at each end of the lamp to protect the terminals from being coated with extrudate and the metallic ferrules of the lamp may be pre-coated with an adherent which, advantageously, may be a heat-activated adhesive. According to another embodiment, instead of using an adhesive, each end of the lamp may advantageously be heated and then immersed in an air-fluidized bed of powdered ethylene vinyl acetate to pre-coat the metallic ferrules of the lamp. The prepared lamp is then introduced into the airlock of the extruder crosshead to receive the cylindrical sheath which adheres to the contours of the lamp.
Advantageously, as the trailing end of the first fluorescent lamp enters the crosshead, a second fluorescent lamp is inserted so as to make the process continuous for a number of successive lamps. At a convenient distance downstream from the crosshead, power driven rollers move the encapsulated lamp to a first cutting position where the extrudate between successive lamp ends is sheared, separating the encapsulated lamps from one another. Further downstream a heated iron is advantageously used to seal the extrudate to the plastic end cap. The silicone tubing used to cover the electrical terminals may now be removed and the coated, shatterproofed lamps may then be packed for shipment.